1. Field of the Invention
The present invention relates to an image sensor, a module and an electronic device including the module. In particular, the invention relates to a linear image sensor used for document reading and for image input for example, a module including a plurality of linear image sensors and an electronic device including the module.
2. Description of the Background Art
A common CMOS (Complementary Metal Oxide Semiconductor) image sensor includes a plurality of photoelectric conversion elements (such as photodiodes for example) for converting an optical signal into an electric signal and a plurality of amplifier circuits provided correspondingly to a plurality of photoelectric conversion elements respectively. The amplifier circuit is configured for example with a source-follower circuit including a MOS transistor.
In the image sensor having the above-described configuration, regardless of whether or not light is incident on the image sensor, an offset signal at a certain voltage level could be output from the source-follower circuit. Further, respective magnitudes of offset signals of a plurality of source-follower circuits could be different from each other. The reason why respective magnitudes of offset signals are different from each other is that characteristics of the MOS transistors could be different from each other due to a factor such as a manufacturing process.
If the above-described phenomenon occurs, respective reference levels of image signals that are output from respective pixels are different from each other. In this case, even if an image is reproduced based on image signals that are output from the image sensor, the original image read by the image sensor could not be reproduced with precision.
For example, Japanese Patent Laying-Open No. 05-048844 discloses an image sensor in which differences in characteristics between source-follower circuits can be reduced. In this image sensor, a voltage that is output from the source-follower circuit immediately after the photoelectric conversion element is reset is held in a capacitor first. Then, as the photoelectric conversion element receives light, the output voltage of the photoelectric conversion element (namely the input voltage to the source-follower circuit) changes. Accordingly, as the output voltage of the source-follower circuit changes, only the voltage corresponding to a change of the output voltage of the source-follower circuit is output from one end of the capacitor. Thus, from one end of the capacitor, an image signal from which an offset component is removed can be taken.
The image sensor disclosed in Japanese Patent Laying-Open No. 05-048844 uses a system in which the capacitor is charged with a signal voltage from the photodiode and the charge of the capacitor is used to convey a signal to the outside. Therefore, a line connecting one end of the capacitor and an output circuit is long. In this case, a parasitic capacitance due to the length of the line is also large. Supposing that the capacitance of the capacitor is C1 and the parasitic capacitance of the line is C2, the voltage of the image signal that is output from the other end of the capacitor decreases to a voltage which is C1/(C1+C2) times as large as the original voltage. In other words, a problem of signal attenuation occurs.
Japanese Patent Laying-Open No. 05-048844 discloses an output circuit for solving this problem. However, the other end of the capacitor is connected by a common line to the output circuit. Therefore, the problem of the signal attenuation due to the long line has not been solved.